.TH nfsslower 8  "2017-09-01" "USER COMMANDS"
.SH NAME
nfsslower \- Trace slow NFS file operations, with per-event details.
.SH SYNOPSIS
.B nfsslower [\-h] [\-j] [\-p PID] [min_ms]
.SH DESCRIPTION
This tool traces common NFSv3 & NFSv4 file operations: reads, writes, opens, and
getattrs. It measures the time spent in these operations, and prints details
for each that exceeded a threshold.

WARNING: See the OVERHEAD section.

By default, a minimum millisecond threshold of 10 is used. If a threshold of 0
is used, all events are printed (warning: verbose).

Since this works by tracing the nfs_file_operations interface functions, it
will need updating to match any changes to these functions.

This tool uses kprobes to instrument the kernel for entry and exit
information, in the future a preferred way would be to use tracepoints.
Currently there aren't any tracepoints available for nfs_read_file,
nfs_write_file and nfs_open_file, nfs_getattr does have entry and exit
tracepoints but we chose to use kprobes for consistency

Since this uses BPF, only the root user can use this tool.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH OPTIONS
\-p PID
Trace this PID only.
.TP
\-j
Trace output in CSV format.
.TP
min_ms
Minimum I/O latency (duration) to trace, in milliseconds. Default is 10 ms.
.SH EXAMPLES
.TP
Trace synchronous file reads and writes slower than 10 ms:
#
.B nfsslower
.TP
Trace slower than 1 ms:
#
.B nfsslower 1
.TP
Trace slower than 1 ms, and output just the fields in parsable format (CSV):
#
.B nfsslower \-j 1
.TP
Trace all file reads and writes (warning: the output will be verbose):
#
.B nfsslower 0
.TP
Trace slower than 1 ms, for PID 181 only:
#
.B nfsslower \-p 181 1
.SH FIELDS
.TP
TIME(s)
Time of I/O completion since the first I/O seen, in seconds.
.TP
COMM
Process name.
.TP
PID
Process ID.
.TP
T
Type of operation. R == read, W == write, O == open, G == getattr.
.TP
OFF_KB
File offset for the I/O, in Kbytes.
.TP
BYTES
Size of I/O, in bytes.
.TP
LAT(ms)
Latency (duration) of I/O, measured from when it was issued by VFS to the
filesystem, to when it completed. This time is inclusive of RPC latency,
network latency, cache lookup, remote fileserver processing latency, etc. 
Its a more accurate measure of the latency suffered by applications performing
NFS read/write calls to a fileserver.
.TP
FILENAME
A cached kernel file name (comes from dentry->d_name.name).
.TP
ENDTIME_us
Completion timestamp, microseconds (\-j only).
.TP
OFFSET_b
File offset, bytes (\-j only).
.TP
LATENCY_us
Latency (duration) of the I/O, in microseconds (\-j only).
.SH OVERHEAD
This adds low-overhead instrumentation to NFS operations,
including reads and writes from the file system cache. Such read, writes and
particularly getattrs can be very frequent (depending on the workload; eg, 1M/sec),
at which point the overhead of this tool (even if it prints no "slower" events) can
begin to become significant. Measure and quantify before use. If this
continues to be a problem, consider switching to a tool that prints in-kernel
summaries only. This tool has been tested with NFSv3 & NVSv4, but it might work
with NFSv{1,2}, since it is tracing the generic functions from nfs_file_operations.
.PP
Note that the overhead of this tool should be less than fileslower(8), as
this tool targets NFS functions only, and not all file read/write paths.
.SH SOURCE
This is from bcc.
.IP
https://github.com/iovisor/bcc
.PP
Also look in the bcc distribution for a companion nfsslower_examples.txt file containing
example usage, output, and commentary for this tool.
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Samuel Nair
.SH SEE ALSO
biosnoop(8), funccount(8), fileslower(8)
